1. Field of the Invention
This invention relates to a projection type image display apparatus, such as a liquid crystal projector, etc., and to be more detailed, relates to an optical unit to be used in a projection type image display apparatus.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2001-154268 proposes an example of an arrangement using a prism type element which performs color separation or color combination of light and a wavelength-selective polarization rotating element in an optical unit for a liquid crystal projector. This publication proposes, as a method of holding the wavelength-selective polarization rotating element, a method of direct, surface adhesion onto the prism type element.
The same publication also proposes an optical configuration in which the wavelength-selective polarization rotating element is separated from the prism type element.
Also as a method of holding and cooling a polarizing plate, Japanese Patent Application Laid-open No. 2002-221758 proposes an art of providing a polarizing plate between a liquid crystal panel and a prism type element, serving as a color combining element, and cooling the polarizing plate by providing spaces at both sides thereof.
However, the following problems occur when a wavelength-selective polarization rotating element is surface-adhered directly onto a prism type element as in the optical unit disclosed in Japanese Patent Application Laid-open No. 2001-154268.
That is, the material of a prism type element is glass and the linear expansion coefficient thereof is approximately 0.4×10−5. On the other hand, a wavelength-selective polarization rotating element is manufactured by multiple layering of polycarbonate and the linear expansion coefficient thereof is approximately 3 to 6×10−5 and thus approximately 10 times that of glass. When in a case where such materials which greatly differ in linear expansion coefficient are fixed together by surface adhesion, a temperature rise occurs due to the driving of the projection type image display apparatus, internal stress will occur at the adhesion surfaces of both elements, giving rise to photoelasticity (birefringence) in the elements and leading to the lowering of contrast of a projected image.
The causes of temperature rise include the absorption of light from the light source by the polarizing plate in the normal driving of the projection type image display apparatus. Other causes include light energy absorption at the dielectric film surface (polarization separating surface) of a polarization beam splitter, which is a prism type element, and light energy absorption by the prism type element itself. A liquid crystal panel which is fixed to the prism type element is also a major light energy absorption source, and the heat from the liquid crystal panel can heat the prism type element by radiation or the prism type element can be heated by conduction of heat via an indirect member for fixing the liquid crystal panel to the prism type element.
Also, though comparatively low, there is light energy absorption by the wavelength-selective polarization rotating element itself since its transmittance is not 100%.
When the prism type element and the wavelength-selective polarization rotating element are heated by such light energy absorption and temperature rise inside the apparatus and photoelasticity occurs in the two elements, the polarization characteristics become distorted and since light which basically should not be made to enter the projection lens is transmitted, the contrast of the projected image is lowered.
Also, if in order to avoid the above problem, the wavelength-selective polarization rotating element is separated from the prism type element and simply held by another member, the optical axis may fluctuate and so-called registration deviation (display deviation in pixel units of a multiple color image) may occur.
For example, as a holding method based on the optical configuration diagram of Japanese Patent Application Laid-open No. 2002-221758, prisms may be fixed strongly to each other to form a unit, and another part, such as a polarizing plate or a wavelength-selective polarization rotating element, may be held separately by a member which holds the prism unit. In this case, when the member which holds the other element with respect to the prism unit undergoes deformation or rotation, the optical element besides the prism type element becomes tilted, thereby causing registration deviation.
A polarizing plate may be adhered directly to a prism type element, which combines light of three colors and with which the influence of photoelasticity is comparatively low (since light which is shifted in polarization due to the photoelasticity which occurs inside the prism type element is absorbed by the polarizing plate, the influence on display deviation of an image is low), to prevent tilting. However, it is not preferable to adhere a wavelength-selective polarization rotating element directly to a prism type element due to the reasons given above.
In a case where the wavelength-selective polarization rotating element is to be held by the member holding the prism element, the wavelength-selective polarization rotating element cannot be fixed strongly since the transmission wave plane will become distorted and the image will be degraded if arrangements are not made to prevent the occurrence of deflection in the wavelength-selective polarization rotating element.